At the present time, much research is being carried out in order to synthesize substitutes for chlorofluorocarbons (CFC's). These substitution products include fluoroalkanes comprising at least two carbon atoms. Among the synthesis processes, the fluorination of halogenated hydrocarbons with hydrofluoric acid in the presence of an antimony-based catalyst is an industrial method of synthesis.
In these processes, the catalytic species consists of an antimony V halide. This catalytic species is prepared either before the fluorination reaction, or in situ from antimony trichloride, chlorine and a fluorinating agent such as hydrofluoric acid, or by direct fluorination of antimony pentachloride. As in any catalytic process, a deactivation of the catalyst is observed, due to the accumulation of impurities (water, metal salts) and to the formation of organic compounds having high boiling points, which leads to a catalytic activity below the threshold of profitability. For this reason, it proves necessary to purge the reactor and add fresh catalyst.
The deactivated antimony-containing catalyst mixture comprises halides of pentavalent antimony (SbCl.sub.x F.sub.y ; 0.ltoreq.x, y.ltoreq.5 and x+y=5) and also halides of trivalent antimony; moreover, this mixture also consists of organic compounds comprising organic reagents, reaction products and, in particular, unsaturated compounds. The presence of hydracids (HCl, HF) and the antimony content depend on the different processes of catalysis.
Treatment of the used catalyst for storage in dumps often leads to large quantities of wastes with a high storage cost. To safeguard the environment and on economic grounds, recovery and recycling of the catalyst are becoming increasingly desirable. However, current processes for retreatment of the catalyst are either too complex to be carried out or lead to a recovered catalyst which necessitates a subsequent treatment before recycling.
Thus, Patent FR 2,281,893 describes a process for recycling antimony in the form of antimony pentachloride. The purity of the regenerated catalyst is high (&gt;95%), but the process necessitates a large number of steps: chlorination of trivalent antimony to pentavalent antimony; fluorination of pentavalent antimony and organic substances having high boiling points with hydrofluoric acid; concentration of the reaction medium; chlorination of the antimony-containing compounds to antimony pentachloride; and distillation of the latter.
Patent DE 2,140,188 describes a process for recycling antimony in the form of antimony trichloride. Here too, the process is relatively complex, since it necessitates a hydrolysis of the catalyst followed by a precipitation of the antimony in the form of an oxide, then dissolution of the precipitate, reduction of pentavalent antimony, precipitation of trivalent antimony, dissolution in hydrochloric acid and finally distillation of antimony trichloride.
Patent DE 2,110,797 claims a process based on a conversion of the used antimony to antimony pentachloride. The reaction mixture is concentrated in the presence of CCl to convert antimony fluorides to antimony chlorides, to remove a portion of the organic substances and to decompose antimony pentachloride thermally to antimony trichloride and chlorine. After removal of a second portion of organic substances by filtration/washing with CCl.sub.4, the antimony trichloride is chlorinated to antimony pentachloride, the latter then being separated by distillation.
Finally, Patent FR 2,320,777 describes a process for recovery/recycling of the antimony-containing catalyst in the form of antimony pentachloride. The crude reaction mixture containing the used catalyst is heated while an inert gas (nitrogen, F 113, etc.) is bubbled through it, with the addition of HF if required, in order to convert the organic substances having high boiling points to fluorinated compounds having lower boiling points. The catalyst is then chlorinated in the presence of chlorine and carbon tetrachloride to convert it to antimony pentachloride; the latter is then recovered by distillation.
In all the patents cited above, the recycling of the antimony-containing catalyst is often relatively complex, but most particularly it necessitates, in all cases, the introduction of intermediary substances (hydrofluoric acid, reducing agents, precipitating agents, chlorine, carbon tetrachloride, solvent, inert gas, etc.) or additional steps (thermal reduction, washing, etc.) which are involved only as regards the recovery and purification of the antimony. These intermediary substances and additional steps are not involved in the fluorination reaction for which the catalyst is used, and no financial return can hence be set against their cost.